1. Field of the Invention
The present invention relates to a temperature phase shift circuit for performing temperature compensation of the phase of a signal or time data in, e.g., an environment with a change in temperature, and an ultrasonic wave coordinate input apparatus using this circuit.
2. Related Background Art
Normally, countermeasures against temperature changes in various electronic equipments which are susceptible to influences of such temperature changes are classified as follows.
(1) When a change in characteristics of an electronic device such as a sensor upon a change in temperature. poses a problem, the temperature is measured, and the obtained data is corrected in accordance with the measured temperature. Particularly, when a high precision is required, a temperature control mechanism is arranged to keep the temperature of the electronic device at a predetermined value.
(2) When a change in characteristics of an electronic circuit, mainly a semiconductor element upon a change in temperature poses a problem, as a first method, a feedback loop is formed in a circuit to perform feedback control, thereby obtaining a predetermined output amplitude. As a second method, reverse characteristics, with respect to temperature, of a temperature compensation passive element (represented by a temperature compensation ceramic capacitor) are utilized to cancel the change in characteristics of the semiconductor element upon the temperature change. The former method is normally used to stabilize the gain of an amplifier or the oscillation frequency of an oscillator. The latter method is generally used when the same purpose as that of the former needs to be attained though the precision need not be so high.
As an apparatus influenced by a change in temperature, an ultrasonic wave coordinate input apparatus will be described below. FIG. 2 is a block diagram schematically showing the arrangement of an ultrasonic wave coordinate input apparatus.
In FIG. 2, an arithmetic operation/control circuit 1 controls the entire apparatus and at the same time calculates a coordinate position. A vibration member driving circuit 2 drives a vibration member 4 in a vibration pen 3. A pulsed driving signal is supplied from the operation/control circuit 1 to the vibration member driving circuit 2. The driving signal is amplified by the vibration member driving circuit 2 with a predetermined gain and then applied to the vibration member 4. The electrical driving signal is converted into a mechanical vibration by the vibration member 4 and transmitted to a vibration transmission plate 8 via a pen tip 5. The vibration transmission plate 8 is constituted by a transparent member such as an acrylic or glass plate. An input operation by the vibration pen 3 is performed by touching the surface of the vibration transmission plate 8 with it.
That is, when the vibration pen 3 is brought into contact with an area (effective area) A indicated by a solid line in FIG. 2, a vibration generated by the vibration pen 3 is input to the vibration transmission plate 8, and the input vibration is detected by vibration sensors 6a to 6d. The detected vibration is converted into an electrical signal by the vibration sensors 6a to 6d, and its waveform is processed by a signal waveform detection circuit 9, thereby extracting the arrival timings of the vibration to the vibration sensors 6a to 6d. A predetermined time offset amount is subtracted from the time lengths from the driving timing of the drive signal to obtain the arrival timings of the vibration. The obtained values are used as vibration propagation times that were required by the vibration to propagate on the vibration transmission plate 8, thereby performing arithmetical processing. With this operation, the distances between the point designated by the vibration pen 3 to the vibration sensors 6a to 6d are calculated. A coordinate position can be calculated from a plurality of obtained pen-to-sensor distances by using triangulation.
The predetermined time offset amount is the sum of a time serving as the same offset amount for all the vibration sensors 6a to 6d, e.g., a time necessary for propagation of the vibration at the pen tip 5 of the vibration pen 3, and a time serving as an offset amount which is different for each of the vibration sensors 6a to 6d, e.g., a circuit delay generated in the signal waveform detection circuit 9 or the response time of each of the vibration sensors 6a to 6d. This time is present independently of the actual vibration propagation time.
The propagating vibration is reflected by the end face of the vibration transmission plate 8. To prevent (or decrease) the reflected wave from returning to the central portion, an anti-vibration member 7 is provided to the side surface portion of the vibration transmission plate 8. The operation/control circuit 1 performs coordinate calculation processing by the vibration transmission timings extracted by the signal waveform detection circuit 9, and at the same time, outputs coordinate data to a display driving circuit 10, thereby displaying the data on a display 11 such as a liquid crystal display unit. The display 11 is arranged behind the vibration transmission plate 8 and displays dots at positions traced by the vibration pen 3. These dots can be observed through the vibration transmission plate 8 (transparent member).
If the ambient temperature around the apparatus changes, the time length from the driving timing to the arrival timing changes accordingly. For this reason, a coordinate portion may be erroneously calculated and output. The main factors contributing to this problem are as follows.
a) In accordance with a change in temperature of the vibration pen 3, the propagation time of the vibration in the pen tip 5 changes to cause a change in time corresponding to the predetermined time offset amount. The pen tip 5 consists of a polyamide-imide plastic enhance the operability, i.e., feeling of writing by the vibration pen 3. In this case, the sound velocity in the material largely changes with respect to a change in temperature.
b) In accordance with a change in temperature of the vibration pen 3, the frequency of the vibration generated in the vibration pen 3 changes. Since the vibration propagating in the vibration transmission plate 8 is Lamb wave, the propagation speed changes in accordance with frequency. For this reason, the vibration propagation time itself changes.
c) In accordance with a change in temperature of the signal waveform detection circuit 9, the amount of time delay due to the circuit also changes. This is because the circuit has characteristics in which the switching speed of the semiconductor element in the circuit largely changes in accordance with temperature. When a semiconductor element having a switching speed at least 100 times that corresponding to the vibration frequency is selected in advance, a change in temperature of performance can be suppressed. However, this cannot be realized from the viewpoint of cost.
In addition to the above three factors, a change in propagation speed due to a change in temperature of the vibration transmission plate 8 (even at the same vibration frequency), or a change in response characteristics of the sensor is also a potential factor, though the influence of such factors is extremely small as compared to the above three factors.
A change in arrival timing due to the above three factors results in a change in the same direction, i.e., when the temperature increases, the arrival timing is delayed, and when the temperature decreases, the arrival timing is advanced. If the vibration generated by the vibration pen 3 has a frequency of about 500 kHz, the arrival timing changes by about 450 ns within a temperature range of 0 to 40.degree. C. If the distance between the vibration pen 3 and the vibration sensor 6 does not change, the arrival timing linearly increases with respect to temperature. The value, 450 ns corresponds to about 1/4 the period of the vibration and is too large to ignore. In this case, the value, 450 ns, is exemplified. However, this value changes when the vibration frequency or shape of the pen is changed in accordance with the use purpose of the apparatus. The value, 450 ns, is therefore only an exemplary value.
Of the factors a) to c), the factor a), i.e., a change in vibration propagation time in the pen tip 5 is considered as the largest factor influencing the coordinate calculation, and accounts for 1/2 or more of the value, 450 ns. Originally, since a plastic has a sound velocity smaller than that of a metal, the absolute amount of a change in propagation time due to a change in temperature increases even when the propagation length does not change. Therefore, although the length of the pen tip 5 is currently minimized, the above difference in propagation time is still generated.
The following methods are used to eliminate the difference in propagation time.
.alpha.) Even when the ambient temperature changes, the predetermined time offset amount is updated to a new value by designating a known input point, thereby correcting the delay time.
.beta.) Vibration sensors which number more than that necessary for coordinate calculation by at least one are arranged, and a change in time offset amount, which is included in the arrival timings of all the vibration sensors by the same amount, is obtained, thereby performing distance and coordinate calculations.
These methods are used as countermeasures against temperature changes, thereby preventing variations of the function of the apparatus itself depending on the variation amount of the physical characteristics of the vibration pen 3 (electroacoustic conversion element).
However, the above-described conventional countermeasures, applied to an electronic apparatus which is liable to the influence of the environmental temperature, have the following drawbacks.
1) In order to compensate the characteristics of an electronic device such as a sensor, a large-scaled operation control system for measuring the temperature by a temperature sensor or the like and performing correction, or a large-scaled temperature control system for keeping a predetermined temperature is required. In any case, the apparatus itself becomes bulky and expensive.
2) The conventional temperature compensation circuit is used to keep a predetermined circuit gain or oscillation frequency or to linearly change them with respect to temperature. Therefore, this temperature compensation circuit cannot compensate the phase or detection timing of a signal.
In addition, the temperature compensation ceramic capacitor to be used has a performance range of .+-.5% even if it is of a high-precision type. For this reason, a compensation circuit constituted by a capacitor prior to selection, i.e., at a low cost becomes a factor for variations in characteristics. Since the temperature coefficient is not very large, several hundred ppm/.degree.C at maximum, the performance range for temperature compensation is accordingly limited.
Furthermore, the conventional ultrasonic wave coordinate input apparatus has the following drawbacks.
1) In the arrangement .alpha.), the user of the apparatus must often designate a known input point, resulting in a complex operation. Additionally, since a known input point is designated by the user, the coordinate calculation precision is degraded.
2) In the arrangement .beta.), at least one more sensor is necessary, resulting in an increase in cost. Additionally, the arrival timing used for calculation of the variation amount of the time offset includes an error such as circuit noise. For this reason, a calculation error is generated in the obtained variation amount of the time offset as well, resulting in a degradation in coordinate calculation precision as in the arrangement .alpha.).